Micromanipulations such as microsurgery require a distortion free magnified and binocular stereoscopic view of its operating field 101. Surgical Loupes, Surgical Light Microscope (SLM) and its digital adaptation, namely, Digital Stereo Microscope (DSM) provide such views of the operating field 101. SLMs have a number of optical elements that provide magnification, working distance and distortion correction. SLMs have an objective lens for collecting the light from the subject, a pair of oculars for viewing the subject and a number of optical elements which enhances the view through magnification and correction of distortion. SLMs are improvements on the classical double barreled binocular microscope to address issues of distortion, limited working distance, limited field of view, and brightness. An exemplary prior art SLM/DSM 100 is illustrated in FIG. 1. To increase magnification, either an objective lens 102 is changed or zoom lens system 104 is moved. The pair of oculars provides views having the required parallax to the right and left eye. SLMs provide instant stereo view for depth perception to the human visual system. Operating fields 101 provided through visual aids such as SLMs, DSMs and surgical loupes should allow a user to perform bi-manual microsurgical manipulations with the subject/objects in the operating field and optionally sharing the operating field with one or more collaborators.
FIG. 2 illustrates the relationship between distance, D, to an object 200 from the lens 202, focal length, f, of the lens 202, separation, B, of the lenses 202 and the disparity |l−r| of the image pixels in the left and right eye views in a stereoscopic view. Field of View (FoV), Depth of Field (DoF), Working Distance (W) and magnification (Z) are inter-related in an optical system. When magnification is increased, the field of view and depth of field is decreased. Various conventional systems and methods have added optical elements to rectify these limitations but the key dependency between FoV and Z remains. Added optical elements also increase the size of the microscope, optical impedance and reduce the brightness. Additional lighting innovations further increase the size, but the main drawback of FoV and Z dependence still remains.
Using the current microscopes, it is not possible to view a location that is not in the optical path through the objective lenses 102 and the oculars 120. Users will need to reorient the object or subject (example, patient) to obtain a view from another angle. In addition, physical construction of the microscope allows very limited numbers of oculars. Optical microscopes do not allow additional content to be added to the view or enhance the view with mensuration or annotation.
Additionally, there are many usability issues in performing micro manipulations using SLMs. A user of a SLM gets visual fatigue because the exit pupil diameter of the ocular lens is very small and lateral movements of the observer causes motion in the field of view. Viewing stereo through surgical microscopes is also a learned skill. It takes significant amount of practice to be able to find the right combination of eye muscle coordination and inter-papillary distance setting of the microscope that allows viewing through both eye pieces at the same time to result in a brain-fusible stereo pair projected on to the retina. This makes stereoscopic viewing through binocular microscope tedious and tiring. Each time, when the eyes are taken away from the binocular eye pieces and brought back; there is a certain degree of eye muscle adjustment to be made before the view becomes stereoscopic. This inconvenience adds to the operational time and operator fatigue.
Another limitation is the narrow field of view. Microscope construction makes use of a large number of optical elements arranged in a barrel. The large number of optical elements narrows the field of view. FoV gets further reduced as the magnification is increased. Narrow field causes the surgeons to lose the visual context frequently. A typical corrective action is to zoom out to bring back the context and then zoom in to achieve magnification while maintaining the tool or tissue in the field of view. This too adds to the operational time and operator fatigue. Surgeons cooperate with human assistants who share the same workspace as the surgeon. The assistant should view what the surgeon is viewing, in order to be of operational assistance. Physical construction of SLM microscopes typically allow only up to one assistant. In addition, each surgeon should have independent control of viewing the same field-of-view by controlling the lighting, contrast and magnification.
Many surgeries last hours and the fixed posture (looking through the eye pieces) contributes significantly to the fatigue experienced by surgeons. In a lengthy surgery, multiple surgeons and assistants may time multiplex. Individual calibration of the microscope should be done to get continuity. Though magnified view is a significant surgical aid, due to the above limitations only very few surgeons are able to perform micro surgery, the surgery that uses microscopes, though there are far more surgeons with excellent surgical skills.
Referring back to FIG. 1, conventional SLMs/DSMs 100 use high resolution and speed imaging sensors such as (Charge Coupled Devices) CCDs 110 to supplement the oculars. The SLM/DSM 100 is retrofitted with imaging sensors at an eye piece 120 and then the view is sent to a stereoscopic monitor 112 via a display processor 114 to view the FoV of an object 120. Specifically, the CCDs 110 can receive the view from beam splitters 122 before the eye piece 120. Surgeon uses stereoscopic viewing techniques such as passive stereo glasses, active stereo shutter glasses and auto stereoscopic displays. Since the imagery is acquired and displayed digitally, these microscopes are called digital stereo microscopes (DSM). A commercial example is the digital stereo microscope by TrueVision™. When using the TrueVision™ microscope, surgeon can either look through binocular barrels of the optical microscope as in the case of the traditional microscope or look away at the stereoscopic screen to view the workspace. The disadvantages of the former approach have already been discussed. In the latter viewing setting, as the display 112 is located elsewhere, surgeon loses the key hand-eye collocation and as a result compromises the hand-eye coordination needed for highly dexterous manipulations under magnified view. In addition, since the basic field of view is captured using the objectives of a traditional SLM, it suffers many of the limitations of the SLM described earlier.
The human visual system has a variable resolution vision. The resolution drops from the foveal vision to the peripheral vision. The visual acuity deteriorates from central (−1, +1) degree visual field to the (−59, +110) degree visual field. Roughly, the (−2, +2) degree visual field has half the resolution of the (−1, +1) degree visual field, (−5, +5) degree visual field has half the resolution of the (−2, +2) visual field and (−12, +12) has half the resolution of the (−5, +5) visual field. However, SLMs or DSMs do not provide any visual aid to match the visual resolution variance. Performing such variable resolution without depth distortion effects is also a challenge.
Stereo microscopes without oculars are another conventional system. These use mirrors that rotate fast to project the right and left view to right and left eye without having an eye piece. A user of this microscope needs to place the users eyes aligned to the projected space to be able to see stereo. While the above system affords some freedom of head movement, the projected space is quite narrow and it is easy to slip out and loose stereo vision. This system also requires training to use and even after training, at each usage there is a task of aligning eyes to the projected space. Peripheral vision can easily distract the eyes from seeing stereo. These microscopes have not found a home in operating theatres yet and are used in the manufacturing industry for inspection of printed circuit boards and other miniature electronic assemblies.